1. Field of the Invention
The present invention generally relates to integrated circuits, and more particularly, to Ball Grid Array (BGA) integrated circuit packages having various ground wiring schemes which provide one or more of the following: a reduced return current loop reduced wire bond inductance and/or improved characteristic impedance and cross talk control between bond wires.
2. Description of Related Art
One type of common package for semiconductor integrated circuits is a Ball Grid Array, often referred to as a BGA package. Typically, a BGA includes a die mounted onto a substrate using some type of die attach material. The substrate is usually made of an epoxy material with metal contacts formed on the top surface. Wire bonds are provided between contact pads on the die and the metal contacts on the substrate. Metal traces along the surface and through the thickness of the substrate electrically couple the metal contacts on the top surface of the substrate to metal balls formed on the bottom surface of the substrate. The metal balls are typically arranged in a grid or array, hence the name Ball Grid Array or BGA package, and are used to mount the package onto a printed circuit board or the like. The balls are aligned and soldered to matching contacts on the printed circuit board. Electrical input and output (I/O), power and ground signals can thus be provided to the chip through the balls, traces, and bond wires to the die encapsulated in the package.
Depending on the complexity of the die, two, three or more rows of bond pads, arranged in a staggered pattern, can be provided around the outer periphery of the die. A ground and power ring are provided on the substrate, around the perimeter of the die, when mounted on the substrate. Wire bonds are formed between individual contacts in the ground and power rings of the substrate and like bond pads on the die to provide power and ground signals to the circuitry on the die. Generally speaking, the ground and power rings are positioned as close to the die as possible to prevent or minimize power loss due to the inductance and resistance of the wire bonds. As a consequence, the I/O contacts are almost always placed outside the power and grounds rings on the substrate surface.
The I/O contacts positioned beyond the ground and power rings on the surface of the substrate creates several problems. With the above-described arrangement, the input/output wires are a distance away from the return ground path wires. As a result, a significant amount of cross talk between the input/output wires may occur. Secondly, the return current loop is relatively large with the I/O contacts placed outside or beyond the power and ground rings on the substrate surface. For a detailed discussion on both cross talk and the return current loop issues, see for example, “BGA Crosstalk”, delivered on the tech on-line forum by Dr. Howard Johnson, Mar. 1, 2005, incorporated by reference herein for all purposes.
A Ball Grid Array (BGA) integrated circuit package having (i) an additional dedicated ground ring on the package substrate which provides a reduced return current loop path to reduce wire bond inductance; and/or (ii) ground wires positioned between adjacent input/output wires on the substrate which provide additional transient current paths among the input/output wires for improved characteristic impedance and cross talk control is therefore needed.